The high-frequency link technology has been widely used in various power supply applications. As shown in FIG. 1, a typical circuit construction of such an UPS includes three sections, i.e., the first section of an AC-DC converting circuit 10 for converting AC of the commercial power to DC, the second section of a DC-AC converting circuit 20 for converting DC to AC output, and the third section of a DC-DC converting circuit 30 for converting the voltage of the backup battery into the desired DC voltage. The circuit further includes energy storing elements C11 and C12, where the energy is stored to provide the inverter with the positive and negative voltages with respect to the neutral line. In case the power supplying condition of the commercial power falls within the predetermined permissible range, the commercial power provides the required energy to the entire device. The AC-DC converting circuit 10 allows the energy storing elements to store energy, and DC-AC converting circuit 20 provides high quality input of energy to a load. When the power supplying condition of the commercial power exceeds the range, or an interruption occurs, the energy is provided by the backup battery. DC-DC converting circuit 30 stores energy in the energy storing elements, while the DC-AC converting circuit 20 provides the load with uninterrupted energy supply. For the purpose of safety, usually the input naught line is directly connected to the output naught line. In addition, in order to reduce the power pollution of the UPS on the electric network, a typical AC-DC converting circuit 10 has the ability of power factor correction, and the circuit with such function can be referred to as power factor correction (PFC) circuit. Moreover, for reducing cost and simplifying the circuit construction, it is desired that the first section and the third section can be combined together.
Recently, a number of PFC circuits for UPS have been proposed. For instance in U.S. Pat. No. 4,719,550 entitled “Uninterrupted Power Supply with Energy Conversion and Enhancement”, a half-bridge high-frequency rectifier circuit is disclosed, the principle of which is shown in FIG. 2. By turning on and off Q21 and Q22, the positive and negative voltages can be obtained on the capacitors C21 and C22, while the waveform and the phase of the input current and input voltage are consistent with each other. However, a drawback of this circuit lies in that Q21 and Q22 cannot be turned on the same time, otherwise Q21 and Q22 will cause a short circuit current between the capacitors Cl and C2, thereby generating a large short circuit current in the system. Another drawback is that another converting circuit has to be used for connecting the battery, as illustrated in FIG. 4, which in turn increases the size and the weight of the system and deteriorates the reliability of the system.
In U.S. Pat. No. 6,069,412 entitled “Power Factor Corrected UPS with Improved Connection of Battery to Neutral”, a PFC circuit of a boost circuit style is disclosed, the principle of which is illustrated in FIG. 3. The basic topology of the PFC circuit is a boost circuit, and the boost and PFC functions are realized by turning on and turning off Q31 and Q32.
There are various methods of connecting the battery in such a circuit. FIG. 5 shows a simple method, whereby the battery is connected to the PFC circuit through a switch S51 and diodes D55, D56 and D57, and the voltage of the battery is boosted to the positive bus voltage and the negative bus voltage through two converters respectively. A shortage of this circuit is that it is complicated because too many power diodes are needed for connecting the battery.